1. Field of the Invention
This invention relates to the preparation of mixed-phased pigments.
2. Discussion of the Background
Mixed-phase pigments are known. Mixed-phase pigments based on iron oxide and chromium oxide belonging to the (Fe, Cr).sub.2 O.sub.3 system have also been described. The last-mentioned products are also used in the form of chrome iron brown, and demand for them in the plastics industry for coloring construction components, eg. window profiles, is growing since they possess excellent fastness properties. However, the tintorial properties, such as particLe fineness, dispersibility and color yield, of the chrome iron brown pigments commercially available to date have not so far completely met the high requirements set by the plastics industry.
Mixed-phase pigments are manufactured in general by reacting the oxide components in the solid state at from 800.degree. to 1400.degree. C. This method of production results in pigments which have a very large particle diameter and extensive sintered fractions which, in spite of milling, give pigments which do not meet higher requirements. The use of particularly finely divided oxides, hydroxides, or other compounds to produce a good mixture in aqueous suspension did not result in any substantial improvement, nor were sufficient improvements achieved by using mixtures, such as hydroxides or carbonates, which were prepared by coprecipitation of aqueous salt solutions.
Processes which lead to homogeneous brown products of the general composition (Fe, Cr).sub.2 O.sub.3, in which decomposed iron and chromium compounds are ignited at fairly high temperatures, are also known. However, such processes, which generally start from the corresponding carbonyl compounds of the metals, are inconvenient and expensive because the starting materials are comparatively difficult to obtain. Moreover, the properties of the brown pigments obtained by these processes do not meet the requirements set.